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					Heartwood is wood that has become more resistant to decay as a result of deposition of
chemical substances.

Sapwood is the younger, outermost wood; in the growing tree it is living wood, and its
principal functions are to conduct water from the roots to the leaves and to store up and
give back according to the season the reserves prepared in the leaves.
Mechanical Properties of Wood

1. Orthotropic Nature of Wood
        Wood may be described as an orthotropic material; that is, it has unique and
independent mechanical properties in the directions of three mutually perpendicular axes:
longitudinal, radial, and tangential. The longitudinal axis L is parallel to the fiber (grain);
the radial axis R is normal to the growth rings (perpendicular to the grain in the radial
direction); and the tangential axis T is perpendicular to the grain but tangent to the growth
rings.

2. Elastic Properties
a. Modulus of Elasticity
       Elasticity implies that deformations produced by low stress are completely
recoverable after loads are removed. When loaded to higher stress levels, plastic
deformation or failure occurs. The three moduli of elasticity, which are denoted by EL,
ER, and ET, respectively, are the elastic moduli along the longitudinal, radial, and
tangential axes of wood. These moduli are usually obtained from compression tests;
however, data for ER and ET are not extensive.

b. Modulus of Rigidity
        The modulus of rigidity, also called shear modulus, indicates the resistance to
deflection of a member caused by shear stresses.
c. Poisson’s Ratio
        When a member is loaded axially, the deformation perpendicular to the direction
of the load is proportional to the deformation parallel to the direction of the load. The
ratio of the transverse to axial strain is called Poisson’s ratio.

3. Strength Properties
a. Modulus of rupture- Reflects the maximum load carrying capacity of a member in
bending and is proportional to maximum moment borne by the specimen. Modulus of
rupture is an accepted criterion of strength, although it is not a true stress because the
formula by which it is computed is valid only to the elastic limit.

b. Work to maximum load in bending- Ability to absorb shock with some permanent
deformation and more or less injury to a specimen. Work to maximum load is a measure
of the combined strength and toughness of wood under bending stresses.

c. Compressive strength parallel to grain- Maximum stress sustained by a compression
parallel-to-grain specimen having a ratio of length to least dimension of less than 11.

d. Compressive stress perpendicular to grain- Reported as stress at proportional limit.
There is no clearly defined ultimate stress for this property.

e. Shear strength parallel to grain- Ability to resist internal slipping of one part upon
another along the grain. Values presented are average strength in radial and tangential
shear planes.

f. Impact bending- In the impact bending test, a hammer of given weight is dropped
upon a beam from successively increased heights until rupture occurs or the beam
deflects 152 mm (6 in.) or more. The height of the maximum drop, or the drop that causes
failure, is a comparative value that represents the ability of wood to absorb shocks that
cause stresses beyond the proportional limit.

g. Tensile strength perpendicular to grain- Resistance of wood to forces acting across
the grain that tends to split a member.

h.Hardness- Generally defined as resistance to indentation using a modified Janka
hardness test, measured by the load required to embed a 11.28-mm (0.444-in.) ball to
one-half its diameter.

i. Tensile strength parallel to grain- Maximum tensile stress sustained in direction
parallel to grain.

4. Other Properties
a. Torsion strength- Resistance to twisting about a longitudinal axis.

b. Toughness- Energy required to cause rapid complete failure in a centrally loaded
bending specimen.
c. Creep and duration of load- Time-dependent deformation of wood under load. If the
load is sufficiently high and the duration of load is long, failure (creep–rupture) will
eventually occur. The time required to reach rupture is commonly called duration of load.
Duration of load is an important factor in setting design values for wood.

d. Fatigue- Resistance to failure under specific combinations of cyclic loading
conditions: frequency and number of cycles, maximum stress, ratio of maximum to
minimum stress, and other less-important factors.

e. Rolling shear strength- Shear strength of wood where shearing force is in a
longitudinal plane and is acting perpendicular to the grain.

f. Fracture toughness- Ability of wood to withstand flaws that initiate failure.
Measurement of fracture toughness helps identify the length of critical flaws that initiate
failure in materials.

5. Vibration Properties
         The vibration properties of primary interest in structural materials are speed of
sound and internal friction (damping capacity).
a. Speed of Sound
          The speed of sound in a structural material is a function of the modulus of
elasticity and density. In wood, the speed of sound also varies with grain direction
because the transverse modulus of elasticity is much less than the longitudinal value (as
little as 1/20); the speed of sound across the grain is about one-fifth to one-third of the
longitudinal value.

b. Internal Friction
        When solid material is strained, some mechanical energy is dissipated as heat.
Internal friction is the term used to denote the mechanism that causes this energy
dissipation. The internal friction mechanism in wood is a complex function of
temperature and moisture content. In general, there is a value of moisture content at
which internal friction is minimum. On either side of this minimum, internal friction
increases as moisture content varies down to zero or up to the fiber saturation point.

Advantages of using timber as a structural material

“Timber building is part of future energy-efficient building. Wood is sustainable, CO2
neutral and a highly effective insulator, creating excellent living conditions. One specific
advantage of wood is its ability to reduce energy use. Timber construction has a higher
heat insulation value than conventional construction methods, even with lower wall
thicknesses. An external wall constructed using timber may have only half the thickness
of a brick or concrete wall, yet provide double the thermal insulation value, while at the
same time avoiding the thermal bridging common with other construction methods.
Considering the growing importance of energy-efficient building methods, timber
construction will play an increasingly important role in the future.”
       Wood is increasingly used in housing, nurseries and schools, religious,
administrative, cultural and exhibition buildings, and halls and factories, as well as in
transport-related construction like bridges, sound barriers, hydraulic engineering and
avalanche control.

       The flexibility of lightweight modular timber construction is particularly suited to
multi-purpose halls because of its ready adaptability.

       Wood is a high-performance material, low in weight, yet high in density, with
excellent load-bearing and thermal properties, and the availability of a wide range of
timbers, each with its own characteristics, means wood can be suitable for most special
requirements.

        Timber construction is typically characterized by a multilayered combination of
different materials which work together as a system to provide optimum stability,
thermal, acoustic and moisture insulation, fire safety and constructional wood
preservation.

Flexibility

The flexibility of timber construction methods makes it easier to vary a building’s
orientation on site, its floor plan, the number of rooms, the interior design and the overall
appearance, while timber’s thermal efficiency means walls can be slimmer, releasing up
to 10% more space than other building methods.


Durability

With good design and correct detailing, structural wood needs no chemical treatment to
achieve a long life. Wood is resistant to heat, frost, corrosion and pollution; the only
factor that needs to be controlled is moisture. Timber construction materials are kiln-
dried to specified moisture levels, removing the need for chemical wood treatment in
interior use. Externally, design elements, such as large roof overhangs and sufficient
distance between timber and ground are important. Timber facades are non-load bearing
and therefore do not require treatment. However, extended life spans can be achieved by
using heat treated timber, special timber qualities, treatments or decorative finishes.

				
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posted:6/11/2010
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